Hi Kellie!
The Campbell text was not the primary source used when writing for this question. Texts can vary in recommendations regarding shoe lifts – when one is needed and internal vs external. We tried to make the options outside of the most common insert values to help you narrow it down to the best answer. I agree with Campbell that intervention is typically not utilized for a LLD <2.5 cm (which excludes the 1.5 cm option). 6 and 8 cm LLD would be far too large to consider an internal shoe lift. This leaves 3 cm as the best answer. I can see the dilemma here because right around an inch is when you consider internal vs external. However, given the information in the question, 3 cm would be the best answer.
Jessica